1. Field of the Invention
The present invention relates to insulated roofing systems, and more particularly to such insulated systems as utilize perlite board with an outer sealing layer of an improved modified bitumen membrane.
2. Description of the Prior Art
Perlite is a volcanic glass, which, as an ore, consists primarily of silica (65-70% by weight), alumina (12-16% by weight), and, most importantly, 2 to 5% by weight trapped water. Rapid heating of the perlite ore to a point just prior to fusion causes the trapped, vaporized water to suddenly expand or puff out the softening perlite particles. A cellular expanded product of extremely low density is produced, and if the expansion is carefully controlled with respect to temperature and time, the expanding perlite forms sealed bubbles. Upon cooling, the air pressure within the bubble is reduced to subatmospheric, thus creating a partially evacuated air vessel--a notoriously poor heat conductor.
The low density, chemical inertness, and heat resistancy of expanded perlite makes perlite a valuable heat insulation material, with it frequently being used as weight aggregate in plaster and concrete. For example, in McCollum (U.S. Pat. No. 2,902,379), a perlite and asbestos agglomerate is taught, which in turn is added to a plaster or cement mixture to form the finished product. When used solely as an insulation material, very little load-bearing capacity is required, e.g., an admixture of perlite and various vegetable or ineral fibers as provided by Denning (U.S. Pat. No. 3,042,578) and a perlite, fiber, and a metallic phosphate binder as taught by Venable (U.S. Pat. No. 3,886,076). When greater strength is required of the perlite construction, admixtures of perlite and cement are provided such as in Gray (U.S. Pat. No. 4,042,406) and in Duvier, Jr. (U.S. Pat. No. 4,518,431) or admixtures of perlite with silicates as disclosed in Vogel, et al. (U.S. Pat. No. 4,298,554) and Samanta (U.S. Pat. No. 4,446,040).
As discussed in Ruff and Nath (U.S. Pat. No. 4,313,997), conventional perlite insulating board is made from an aqueous slurry of fibers, sizing, and expanded perlite. The slurry is thereafter formed into boards by a Fourdrinier process involving alternating sequences of suction and pressure, followed by a drying operation, typically in an oven. After being dried and cut into commercially-sized boards, the perlite board is ready for installation in a variety of construction applications. When used as a roofing insulation material, the perlite board is laid directly on and attached to a roof deck in a manner responsive to the local building code requirements. The principal design criteria with respect to its attachment to the roof relates to wind-uplift resistance. Weather-proofing is provided by applying an asphaltic or bitumen-based product to the upper, exposed surface of the perlite board. The conventional rubberized bitumen membrane available in the roofing industry is generally known as "modified bitumen membrane." The modification consists of reacting the bitumen with either Atactic Polypropylene (APP) or Styrene Butadiene Styrene (SBS) modifiers. Reference to "bitumen membrane" or "bitumen" will mean "modified bitumen" unless otherwise specified.
Attachment of the bitumen membrane is somewhat hampered by the poor peel-resistance of the perlite board. In order to provide a better bond between these two materials, ones that differ greatly in resilience and flex characteristics, an intermediate glass base sheet is placed between the perlite board and the bitumen outer layer. The glass base sheet is anchored to the perlite board either by mechanical fasteners or by an intermediate layer of hot asphalt, which is mopped onto the perlite board surface prior to laying down the base sheet. Attachment of the outer bitumen member then proceeds utilizing on-site portable propane torches to soften the bitumen layer contemporaneously with its placement on the glass base sheet.
Although the foregoing perlite/glass base sheet/bitumen construction performs adequately as an insulating roofing system, a number of advantages could be realized by eliminating the requirement for the base sheet. Not only does the base sheet add an additional materials cost and additional weight to the structural design considerations, but the added labor required to install the base sheet, including the hot asphalt coating process, considerably adds to the cost of roof installation. Despite the many advantages of eliminating the base sheet intermediary layer, its removal has proved impractical to date due to the structural weaknesses posed when combining only the perlite and bitumen elements.
The internal fiber network in perlite board does not provide significant internal resistance to shear and peeling stresses, a characteristic of most pressboard constructions. Thus, one of the functions of the glass base sheet is to reduce the tendency of the asphalt-bitumen sealing layer to peel back and separate from the perlite top surface. To the extent that this structural weakness of perlite has been addressed previously by a series of related Miscall, et al. patents, the solution has been to reinforce the perlite board by adding various materials to the perlite slurry during the manufacture of the pressboard. Such materials include: fibrous materials (U.S. Pat. No. 2,634,207), asphalt binders (U.S. Pat. No. 2,634,208), and asphalt-impregnated fibers (U.S. Pat. No. 2,626,864). The latter two asphalt compositions are said to greatly increase the wet strength of the perlite board, however the addition of the asphalt to the normal perlite board almost doubles the weight of the board, and its effect on peel strength, the important criteria in the roofing system, is not discussed.
A second problem that is posed by removal of the glass base sheet relates to heat damage or scorching of the perlite during installation of the outer asphalt layer. As described previously, the bitumen material is heated by a propane torch just as it is being pressed against the installed perlite board. The open flame will frequently also overheat or scorch the top surface of the perlite board, resulting in a further significant weakening of its peel-resistance. The previously available techniques for controlling flame damage in roofing insulation systems primarily addressed the inflammability of the asphalt. The majority of solutions in this area attempt to reduce the tendency of asphalt to burn and/or flow under heat by admixing various other constituents with the asphalt, such as finely divided mineral fibers in Greider, et al., (U.S. Pat. No. 2,610,928), glass filaments in Schuetz, et al., (U.S. Pat. No. 2,893,889), and halogenated or organic phosphate compounds in Koons (U.S. Pat. No. 3,342,614). In addition to these admixture approaches, Donegan (U.S. Pat. No. 2,782,129) provides a sandwich construction of unexpanded vermiculite between asphalt and asbestos layers. The vermiculite is added for its property of expanding under heat, which is intended to disrupt the flow of burning asphalt, and in this manner prevent the spread of the fire. As noted, none of these teachings relate to protection of the perlite board surface from overheating.
In addition to providing protection to the perlite board, the glass base sheet also provides structural backing to the outer bitumen layer. Without the base sheet, the adhesive quality of the bitumen layer to the perlite and the general resilience of the bitumen material to the normal stresses imposed on roofing insulation must all be structurally accomplished by the bitumen material. The reinforcing filler materials of Fasold, et al. (U.S. Pat. No. 2,555,401), such as slate flour or limestone dust are of little help in providing structural strength, and are added to inhibit the flow of asphalt should it overheat. Of more relevance, albeit in a roadway context, are the additions of various fibers in Gallagher (U.S. Pat. No. 2,507,629). In Zaadnoordijk (U.S. Pat. No. 3,457,136) reinforcing fibrous materials are added to the bitumen material, with the fibrillary products either loose (e.g. individual fibers) or woven or knitted products. The more densely woven fabrics are suggested for large scale uses such as roofing, however there is no discussion as to the effect of such woven fibers on the adhesion of the bitumen layer to the substrate.